May 26, 2023

The evolution of laser welding technology

Laser welding technology has progressed to become the process of choice for metal fabricators and manufacturers because of its mind-boggling variety of applications.

Editor's note: The following is based on "Introduction to Industrial Laser Welding," presented by Tom Kugler, fiber systems manager, Laser Mechanisms Inc., at FABTECH, Sept. 13-16, 2021, Chicago.

Laser welding has permeated high-end, precision metals manufacturing. The technology fills a vital role throughout automotive, medical device manufacturing, and in parts for aerospace and precision electronics. It's now showing up in more places than ever, from the largest OEM to the precision sheet metal job shop.

As laser welding has evolved, it has become extraordinarily flexible. The tremendous variety of welding that lasers can perform is truly mind-boggling. Understanding how lasers accomplish all this starts with knowing the fundamentals—how a beam of light fuses two metals together.

Metals, in general, are very reflective to light. A laser concentrates and focuses that light to overcome the reflectivity. When enough energy from the beam is absorbed, the metal starts to liquefy.

All this starts when the optics—either a curved mirror or a curved-surface lens—focuses light down to a spot size that can range from tens to a few hundred microns in diameter. Such focusing creates extreme power density.

What transparent optics to use depends on the laser and its wavelength. CO2 lasers emit a 10.6-micron wavelength. Standard glass isn't transparent to that, which is why such lasers use alternative lens material like zinc selenide (ZnSe). One-micron lasers—including fiber, disk, and YAG—use fused silica or glass.

ZnSe lenses focusing the 10.6-micron beam of a CO2 laser have excellent heat conductivity, which makes the optics a little more forgiving to debris. Unfortunately, there's no cost-effective material that exhibits similar heat conductivity with the 1-micron laser, which means the focusing environment must remain clean and have good-quality glass or fused-silica optics.

Welding applications that require high laser powers might create some unavoidable debris. In these cases, mirrors are used to focus the beam instead of transparent optics. Focusing mirrors are common in CO2 laser welding applications using 5 kW or more of laser power. One-micron lasers, including fiber and disk, also use mirrors for higher laser powers. A common setup entails a beam (horizontal to the working surface) hitting a parabolic mirror that reflects the beam downward.

Laser optics focus the raw beam diameter to create a depth of focus, where the beam has enough intensity to process material. The narrowest point on the beam waist is the spot size. The focal length is the distance between the lens and the focal point(see Figure 1).

All these variables interrelate. The shorter the focal length, the smaller the spot size, and the shallower the depth of focus. And each of these parameters can be adjusted to optimize a welding process. For instance, extending the focal length can change the focus position and increase the depth of focus, which can increase weld penetration.

FIGURE 1. Variables like the beam diameter, depth of focus, spot size, and focal length all interrelate.

Another factor is beam quality, or the innate focusability of the laser beam. This can't be adjusted—it varies by the laser's type and design—but the parameter does affect how one dials-in the overall process. Lasers with the highest beam quality are called single-mode lasers, which have a purely Gaussian or TEM00 beam with a power density profile that is highly intense in the center and less intense near the edges. The high beam quality helps achieve a greater depth of focus, which in turn opens up a host of processing possibilities.

All common laser types have single-mode versions with high beam quality, but the impact of that high beam quality depends on the laser wavelength. A CO2 single-mode laser at 10.6 microns will have a spot size that's 10 times larger than a fiber laser with a 1-micron wavelength. In general, a shorter wavelength also means a smaller focus spot size.

Again, the whole point of focusing is to overcome the metal's natural reflectivity. Liquid metal absorbs more light energy than solid metal, so when metal enters its liquid phase, energy absorption increases greatly, so much so that it starts to turn the liquid weld pool into a concave shape. That concave shape tends to steer the energy to the center of the weld pool. Once the weld pool becomes deeply concave, it starts absorbing most of the laser energy and reflecting only about 5%. The point at which a metal's initial reflectivity drops to 5% and less is when the process is coupling into the material.

In one sense, laser welding is like bad laser cutting. Instead of removing metal, it's liquifying it in a controlled way. Like in cutting, a laser can use more power to weld faster and thicker. But the process doesn't rely on the aerodynamic advantage of the assist gas flow, evacuating molten metal, nor can it leverage the burning reaction of iron and oxygen. Instead, good laser welding should achieve a controlled melt and often makes use of gases to prevent extensive oxidation.

Material hardness doesn't matter. It's easier to laser weld titanium and superalloys than aluminum. Conversely, reflectivity and thermal conductivity matter a great deal because they all affect how a particular metal absorbs energy from the beam. Materials with very good heat conductivity, such as gold and silver, can present challenges in laser welding. Heat-sink materials like copper, which have high thermal diffusivity (how well a material disperses heat) can be challenging too. That said, modern fiber and disk lasers have enough power density in their beams to overcome these issues.

Unlike laser cutting, laser welding also introduces more metallurgical considerations. Laser cutting turns one piece into two. Laser welding involves metallurgical factors like strength, porosity, brittleness, and microcracking.

Laser welding produces three common types of melt pools: a shallow one produced by conduction-mode welding; a deep, narrow depression created by keyhole-mode welding; and a momentary depression (usually somewhere in between the keyhole and conduction mode) created by a penetration-mode weld, which usually uses a pulsed laser (see Figure 2).

Conduction Mode and Keyhole Mode. Those who know gas metal arc welding (GMAW, or MIG) are familiar with the conduction-mode melt pool and its semicircle cross section. A small laser spot size heats the part just enough to create a melt. The heat conducts from the center of the pool outward, so the pool is hotter in the center and cooler on its edges.

Keyhole-mode welds are just the opposite. Here, the laser has enough intensity to take the liquid metal to its boiling point and expel vaporized metal off the surface at high velocity. The vaporizing metal pushes the liquid metal downward, creating a narrow keyhole (see Figure 3).

That keyhole effectively creates a kind of channel for the laser beam, which changes how it heats and melts the surrounding metal. A weld keyhole might be 10 mm deep but only 1.5 mm wide, and so to achieve a weld, the process need only melt and resolidify the metal surrounding that 1.5-mm keyhole.

FIGURE 2. Conduction-mode welding (left) creates a wide, shallow melt as heat is conducted from the center of the pool outward. Penetration-mode welding creates a weld pool deeper than a conduction-mode weld can, but it isn't as narrow and deep as a keyhole-mode weld.

Contrast this with conduction mode welding. The laser produces a pool that might be 10 mm deep, but heat from the beam conducts outward to create a 20-mm-wide weld pool where all the metal needs to be liquified and resolidified. This doesn't make conduction-mode welding inherently bad, of course. It's just used to achieve different goals, such as cosmetically perfect corner joints and welds in thin materials. Beyond welding, the conduction mode is used for laser cladding—effectively achieving very low dilution between the clad and base material—as well as additive applications.

Penetration Mode. Penetration-mode welding uses pulsed lasers, which have high peak powers but low average powers. For instance, a pulsed laser with a 150-W average power might have a peak power of 1,500 W. Think of striking a nail with a hammer. If you just place the hammer on the nail head, nothing happens; that would be like trying to weld with only 150 W of power. If you swing the hammer and hit the nail in the right way, it can go all the way in after just one hit; that's pulse welding with high peak power.

Penetration-mode welding doesn't create a narrow depression like keyhole-mode welding, but it can create a weld pool that's deeper than it is wide. It also helps control heat input while creating a weld pool that's much wider than a keyhole.

Pulses can be adjusted and shaped for the application. For instance, a shaped pulse is a temporal shape in which the laser's peak power is adjusted over time. This often is used to slow the cooling rate and minimize cracking in material with high carbon content. Other shaped pulses enhance the initial spike, increasing absorption in aluminum and other highly reflective materials. Sometimes initial pulses are used to clean the material surface of debris, oxides, or oils before subsequent pulses create the melt pool and commence welding.

The keyhole's stability is important, especially in partial-penetration welds. In fact, many applications specify full penetration to mitigate those keyhole stability issues.

Sometimes, due to the joint design or other part characteristics, a full-penetration keyhole just isn't an option. A partial-penetration keyhole, though, has a greater chance of wandering—moving up and down as the weld progresses. This movement can leave voids that get sealed over with liquid, creating a pore.

The main concern with 1-micron lasers is beam scattering caused by soot that floats up during welding. This changes the focus spot and reduces the laser power. The keyhole itself might move to the left or right, depending on where the concentrations of soot are. Such movement hinders consistent metal vaporization, which can ultimately cause the keyhole to collapse.

The right gas flow helps here, evacuating impurities and other unwanted elements from the weld zone. When using a fiber or disk laser, jets of assist gas move soot away from the weld zone, often into a fume collection area.

CO2 laser beams don't interact with soot, but they do interact with the plume on top of the weld. The problem starts with the way the 10-micron beam interacts with the plume's free electrons. Once the plume absorbs enough photons, it becomes a white ball of plasma that effectively stops the laser weld. To avoid this, laser welding systems incorporate gas jets that push the plume toward the solidified metal trailing the weld zone.

Because its liquid phase is very short-lived, laser welding induces very little oxidation, which means shielding gas often isn't necessary. Still, some applications, especially in the medical industry, require nearly zero oxidation, and so these laser welding setups often use some kind of shielding gas.

FIGURE 3. In keyhole-mode welding, the beam vaporizes metal to create a narrow depression, either partially or completely through the joint. Minimizing turbulence is key. Turbulence in the keyhole causes instability, which causes liquid metal to seal over voids and create pores.

In many cases, a laser welding application might not require a shield gas, but it does require a weld assist gas, which helps evacuate impurities and unwanted elements like soot from fiber laser welds and plasma plumes from CO2 laser welds. Some applications do use gas as a kind of shielding that suppresses plasma-plume formation. Others use air knives that blow sparks and other debris away from the sensitive welding optics.

Much of laser welding occurs without filler metal, but some applications require it. Filler metal is usually added either to overcome a certain gap or for metallurgical reasons, such as to get around cracking problems.

A nickel filler can overcome cracking problems in certain iron-based alloys and stainless steels. For aluminum, a high-silicon 4000 series filler, like 4047, is sometimes used to weld two 6000-series aluminums together.

Regarding acceptable gaps between base metals, the traditional rule of thumb is to not have a gap that's larger than 10% of the thickness of the thinnest base material. It's just a general rule and can change depending on the material thickness and application. New laser technologies, though, are allowing for larger gaps, which is where beam manipulation comes into play.

Those producing tailor welded blanks face a challenge with every butt-joint weld they make: They’re welding two base metals of different thicknesses. To optimize the process, some have employed laser welding with dual optics in which a prism splits the laser beam into two focus spots. Power can be adjusted from one spot to the other for optimal results, overcoming any excessive gap as well as challenges that come with welding two different material thicknesses.

Similar setups can produce multiple focus spots, either using a prism or faceted mirror, in continuous welding or in a spot welding setup with a pulsed laser. Some welding heads have prisms that can produce three or even four spots simultaneously.

Special heads with diffracted or sculpted optics take the laser output and create a rectangular focus with an even power density. This can work well in some welding applications, but it's more common in heat treating and cladding, especially wire-fed laser cladding applications that require precise levels of dilution between the clad and base metal and high rates of deposition. Some can deposit material at rates of up to 20 kg an hour.

Other applications use galvanometer-driven mirrors to move the focus spot faster. This is common for remote laser welding setups (where the focus distance can be a meter or more) in which galvos move the beam spot from one area to the next nearly instantaneously. Others use optomechanical devices to move the spot, such as a setup in which rotating wedge prisms creates a fast-moving circular path (see Figure 4).

Certain advanced applications move the spot in a small, precise, circular path to create a larger spot, and others scan the beam to create a larger interface between the weld and base metal. This motion often works in conjunction with pulsed lasers with high peak power that help break down reflectivity, such as when welding copper to aluminum.

Yet another recent advancement is laser stir welding (LSW) or wobble welding, a process that manipulates the beam in a continuous circular or other path, designed to smooth the weld surface, increase the width, and eliminate porosity. At a high laser power and slow rotation speed, LSW creates a continuous molten pool with a large melt zone, allowing gas to evacuate and liquid to "heal" the voids (see Figure 5).

FIGURE 4. Two wedge prisms rotate to send the laser beam in a circular path.

In some LSW applications, the beam rotates so quickly that the weld metal literally solidifies right behind it. In these cases, the aim isn't to increase weld strength or create a large melt pool and eliminate porosity, but instead to dial in the resistance characteristics between base metals. Welding in this way minimizes heat input while increasing the weld cross section, which lowers resistance.

Today, laser welding is synonymous with quality. As just one example, some of the most advanced single-mode systems have created precise keyhole welds that, when their microstructure is examined, don't look like welds at all. Only the faintest line exists between the base metal and melt pool. Such quality came from a single-mode fiber laser with an extremely small spot size combined with a very high depth of focus. These welds just weren't possible until recently.

Over the years, lasers have made the previously unweldable weldable, and they’ve made the previously time-consuming and arduous processes simpler and faster. Conduction-mode welded corner joints come to mind. Lasers weld them in one pass, and the workpieces flow directly to final assembly without any grinding or polishing. They look perfect as is. The welding itself might be a little faster, but it's the quality that makes the laser really shine.

FIGURE 5. Laser stir welding moves the beam in a circular path to create a wider weld.